1. Field of the Invention
The present invention relates to an image-sensing device having a plurality of output channels for use in a video camera which can obtain a moving picture image and a still picture image.
2. Description of the Related Art
A solid state image-sensing device converts optical image into an electric signal, and is used for video camera or digital still camera. As to the image-sensing device, resolution of a picture depends on a number of pixels in the image-sensing device. Increasing a number of pixels can capture higher resolution images. Generally, a digital still camera uses an image-sensing device having more pixels than a conventional video camera to obtain higher resolution still picture images.
Recently, a video camera also adopts the function of digital still camera that obtains a still picture image in addition to a moving picture image. Accordingly, the image-sensing device for such a video camera requires more pixels than an image-sensing device for used in a conventional video camera which does not have the function of digital still camera. In addition, a number of pixels in an image-sensing device is increasing every year to obtain higher resolution pictures.
FIG. 1 is a partial enlarged view of a conventional solid state image-sensing device. In FIG. 1, an image-sensing device is composed of a plurality of photoelectric converters 31 which convert optical image into an electric signal, a plurality of vertical transferring CCDs (hereinafter referred to as VCCD) 32 which transfer an electric charge from the photoelectric converter 31 towards the vertical direction, a horizontal transferring CCD (hereinafter referred to as HCCD) 33 which shifts the electric charge from the VCCD 32 towards the horizontal direction, and an amplifier 34 to amplify the electric charge to be outputted from the HCCD 33.
A timing generator not shown provides a driving pulse signal in every one field period of television signal to the image-sensing device. The electric charge in the photoelectric converter 31 is transferred to every adjacent VCCD 32 by the timing of this driving pulse.
The VCCD 32 shifts the electric charge towards the HCCD 33 one by one stage of the VCCD 32, and the HCCD 33 outputs the electric charge through the amplifier 34 when the VCCD 32 transfers one stage of the electric charge to the HCCD 33. Accordingly, a shift velocity of electric charge in the HCCD 33 is set to a velocity which outputs all the electric charges through the amplifier 34 every time the electric charge is transferred from the VCCD 32.
In the VCCD 32 and HCCD 33, as a number of pixels increases horizontally and vertically, a number of stages to shift the electric charge also increases. Consequently, the clock frequency for transferring electric charge should also be increased and set faster as a number of pixels increases.
However, the increase of the clock frequency also increases power consumption, a heat problem and deterioration of signal-to-noise ratio. In view of the problem described above, there provided an image-sensing device shown in FIG. 2.
FIG. 2 is a plan view of an image-sensing device in accordance with the related art. In FIG. 2, the image-sensing device is composed of a pixel area 41 including a photoelectric converter and a VCCD, two HCCDs 42 and 43 for shifting an electric charge transferred from a VCCD in the pixel area 41, two adders 44 and 45 to add a reference signal on the electric charge to be outputted, and two amplifiers 46 and 47 to amplify the reference signal and the electric charge for predetermined level.
The electric charge transferred from the VCCD in the pixel area 41 is supplied to the HCCD 42 and the HCCD 43 respectively and then shifted towards the horizontal direction. Accordingly, the clock frequency necessary for shifting an electric charge in the HCCDs 42 and 43 can be set to half a frequency of using only one HCCD.
Generally, a pixel size of an image-sensing device becomes smaller as a number of pixels becomes bigger. The smaller pixel size deteriorates the transferring efficiency of an electric charge from VCCD to the HCCD 43 and resulted in affecting the quality of the picture.
Especially for a recent video camera, the size of camera itself is miniaturized including an optical mechanism that the size of the image-sensing device also becomes very small (such as ⅓ inch size or ¼ inch size, etc.). To have lots of photoelectric converters in a smaller image-sensing device may deteriorate the transferring efficiency of an electric charge which affects the picture quality.